Catheters serve a variety of functions in medical procedures. In addition to the dilatation of clogged arteries, catheters are used to deliver drugs or other agents to sites in the cardiovascular system, the urethra, bladder, prostate, rectum and central nervous system, such as along the spinal column. Other applications include the delivery and placement of devices, such as a thrombolytic filter.
A variety of thrombolytic filters are available. When positioned, the filters, which are typically in the shape of an umbrella or mesh, expand across the blood vessel. Access to a desired site is often percutaneous, however. The catheter must also often be advanced through narrow vessels or lumens with tortuous turns. A narrow catheter is thus preferred for ease of insertion and advancement. Thrombolytic filters must therefore be capable of being delivered through a narrow catheter shaft.
Such filters typically comprise wires of stainless steel in the desired configuration, compressed for delivery through a catheter. When released from the catheter, they expand across the blood vessel. Shape memory alloys have also been used to form expandable devices which can be delivered in a compressed state and are activated by temperature, for example. The filter can be expanded by a balloon, as well. Some devices include hooks to engage the vessel wall, securing the filter in position. Since thrombosis is most common in the lower extremities, such devices are usually implanted in the inferior vena cava to prevent thrombolytic material from entering the heart, where it could be pumped to the lungs, causing a life threatening pulmonary embolism. Thrombolytic material can also cause a cerebral embolism or myocardial infarction.
Use of metal alloys and wires, however, particularly those with hooks, can damage the tissue of the lumen or vessel, causing thrombosis. The filters may also be difficult to place and remove, particularly when filled with thrombolytic material. Some devices occlude too much of the blood vessel, preventing adequate blood flow. The use of such devices often requires the oral administration of anticoagulants, as well.
A commercially available thrombolytic filtration product, the Prolyser.TM. from Cordis.RTM., comprises an outer catheter shaft of fluoropolymeric material with longitudinal ribs. An inner catheter shaft runs axially through the outer shaft. The inner shaft is attached to the distal end of the outer shaft, and can be retracted. Retraction of the inner shaft brings the ends of the longitudinal ribs closer together, causing the longitudinal ribs to flare outward toward the walls of a vein. Thrombolytic material flowing through a vein can be caught by and trapped within the flared longitudinal ribs. The inner shaft includes ports for the delivery of lytic agents within the region encompassed by the ribs. The lytic agent may be too dissipated when it reaches the ribs, however, to adequately dissolve thrombolytic material caught between the ribs.